The primary goal of this research is to examine the development of toddler self-regulation from 24 to 36 months using a within-family, sibling design and to address changes occurring in the family that coincide with the development of early self-regulatory competence. The study plan involves adding longitudinal follow-ups to an existing study examining changes in the family following the birth of a second child when the second born turns 24 and 36 months of age. Specifically, families participating in the Family Transitions Study (FTS) will be recontacted when their second child is 24 and 36 months old to assess family dynamics and the social and emotional development of both siblings in the family. The 24 and 36 month timepoints extend data collection from six prior timepoints (Prenatal, 1, 4, 8,12 and 18 months) that allows the examination of complex change within the family system for two parents (mother, father) and two siblings (older, toddler). The research proposes to: 1. Identify change trajectories in the second born child's self-regulation from 18 to 36 months of age;2. Move beyond between-family, mother-child research, and include fathers and older siblings in an effort to examine within-family processes that are important for the development of the toddler's early self-regulation. 3. Simultaneously model change in the older sibling's problematic behavior from pre-birth to 36 months after the sibling birth and relate it to change in the toddler sibling's development of self-regulation from 18 to 36 months and;4. Examine whether changes within the family (parental depression, parenting, sibling relations, marital change) during the first three years after the birth act as shared or nonshared family risk factors that predict the development of sibling outcomes from 18 to 36 months. The proposed research is unique in that it includes a large sample of 180 families;followed longitudinally across eight timepoints;includes mothers and fathers, and two siblings;and examines within-family processes not examined to date in the development of early self-regulation. This study will further our understanding of the family processes that predict difficult toddler behavior during the "terrible two's" and aid both professionals and parents in preventing child behavior problems in early childhood.